Mice ablated for nonmuscle myosin heavy chain (NMHC) II-B develop major structural abnormalities in their hearts and brains and die by embryonic day (E) 14.5. These abnormalities include a ventricular septal defect (VSD), mislocation of the aorta which emanates from the right ventricle and a defect in cytokinesis in the heart. The brain defects include abnormal migration of specific groups of neurons (facial and pontine) and a severe hydrocephalus. Cardiac myocytes are unique in that they do not express NMHC II-A, and neuronal cells are enriched for NMHC II-B, which could explain the phenotype observed. The purpose of the present experiments was to see whether expression of NMHC II-A could rescue the defects due to the loss of NMHC II-B. We used GFP-tagged human NMHC II-A cDNA to ablate NMHC II-B by inserting it into the first coding exon for II-B, thus placing GFP-NMHC II-A under control of the endogenous II-B promoter. Interestingly, we found that 20% of the homozygous mice survived beyond 4 months of age without evidence for hydrocephalus nor did they manifest obvious cardiac defects. However, they did show abnormal migration of facial neurons and pontine neurons by histological analysis. Immunoblot and immunofluorescence analyses confirmed no expression of NMHC II-B in the heart, lung and brain, and demonstrated significant expression of GFP-NMHC II-A in these organs. Analysis of mice between E12 and birth revealed that most of the mice (80%) died between E12 and E16.5 showing defects similar to those found in B-/B- mice. These included a ventricular septal defect and binucleated and enlarged cardiac myocytes. However, none of these mice had an aorta that originated in the right ventricle in contrast to II-B ablated mice. The few mice that survived to adulthood developed dilated cardiomyopathy accompanied by myofibrillar fibrosis starting about 3 months of age. Abnormalities included an increase in the heart/body weight ratio, myofibrillar hypertrophy and re-expression of cardiac fetal proteins such as atrial natriuretic peptide and beta-cardiac myosin heavy chain. Thus, we demonstrate that nonmuscle myosin II-B plays an important role during mouse development that cannot be substituted for by myosin II-A.